Standard earthing design - are they valid anymore?

A number of organisations across the UK recommend a standard earthing design when building smaller scale substations. The key driver behind these is one of cost. Nobody wants to pay more than they have to. For an HV connection using a templated approach to meet Earthing Standards compliance kind of makes sense - or does it?

So, the question is - does installing a standard earthing design meet your obligations under the Electricity at Work regulations?

Regulation 8 requires that:

Precautions shall be taken, either by earthing or by other suitable means, to prevent danger arising when any conductor (other than a circuit conductor) which may reasonably foreseeably become charged as a result of either the use of a system, or a fault in a system, becomes so charged; and, for the purposes of ensuring compliance with this regulation, a conductor shall be regarded as earthed when it is connected to the general mass of earth by conductors of sufficient strength and current-carrying capability to discharge electrical energy to earth.

In my opinion, the key part of this extract is the last phrase: i.e. “connected to the general mass of earth by conductors of sufficient strength and current-carrying capability to discharge electrical energy to earth.” WHY?

Standard earthing design - in a non-standard world

You’ve checked that the conductors are of the correct ampacity - surely that’s enough, right? WRONG! Let’s look at the last 5 words -discharge electrical energy to earth.

If you’ve already checked that the conductors can transport the energy away from the location of the fault, that’s great. But if it has nowhere to go, then it’s almost pointless.

As mentioned previously, the key underpinning to any electrical earth system is a soil resistivity study. I.e. determining the capability of the soil to absorb the electrical energy dissipated by the earth (soil conduction). As a result, the earth system designed for a substation next to a river is unlikely to comply with standards, if the substation is actually installed at the top of a mountain. Owing to the different geology likely to be observed.

Since 2010, compliance for HV earth systems has been referenced to BS EN 50522. Though most DNOs refer to the older ENA TS 41-24. Both of these standards define compliance in terms of step and touch voltages, i.e. safety from the system. A compliant earthing system is one where step and touch voltages are kept below the thresholds determined by the above standards, instead of the redundant standard earthing design - “less than 1-Ohm rule”.

Once you know the soil resistivity for the site, you can start thinking about the electrical earthing system design. Questions to ask are:

How much energy is there likely to be when a fault occurs? What is the fault level?

What other routes can the energy take? Is there a cable sheath? Is there a CPC with the cable?

How big is the substation?

What other land is available to deploy an earth electrode into?

If any single one of these points is based on an assumption. The design is either way over the top, so you’re installing much more copper than you need to. Or worse, the design is insufficient, and you could be sending your electrical maintenance staff into a danger area when they go to work in your substation.

We’re all passionate about delivering right-sized designs at GreyMatters. Earthing Design that ticks the safety compliance boxes, and also keeps the accountants/project leaders happy.

If someone working in your substation were to get injured and the substation had an insufficient earth system, your only defence is that you took all reasonable steps and exercised all reasonable due diligence. Sure, an Earthing Study is a few thousand pounds, but if someone were to die because the earthing system you installed wasn’t up to scratch, the damages alone would be significantly more.

shoe-horning an 'off the peg' safe design into a standard specification

So historically, shoe-horning an 'off the peg' safe design into a standard specification to reach a set resistance value across the UK may have worked before 2010, but now, the bar has been significantly raised.

Feeling uncertain? Got any difficult questions werring around? If so, we have an offer to help set your mind at rest.

For new UK-based customers, for a flat fee of 5k, we will visit one of your small (typically a GRP substation in a box) HV substations, do a soil resistivity test, measure the earth resistance, and put together an Earthing Study using the latest most powerful CDEGS software, that either sets your mind at ease, that the earth system is compliant with standards. Or, tells you the next steps to bring the substation into compliance (terms & conditions apply - for more information, please get in touch).

If you've got a problem you'd like us to look at, why not drop us a line on LiveChat?

Hugh is a Research Engineer at GreyMatters, joining in 2014. He graduated from the University of Bath, sponsored by the IET's Power Academy scheme. Hugh has a strong technical background with National Grid working on T&D schemes at voltages from 132 kV to 400 kV. Outside of work, you can find him cycling and fly fishing, or backstage at various local amateur dramatic companies.

"Working with GreyMatters has totally changed my expectations of a professional engineering consultancy firm … for the better! Technically, they were on the money! More importantly however ... they kept me updated throughout the entire project, which was so refreshing, and therefore quickly put my mind at ease that things were being taken care of." (C. Turgis)